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456
CHEMICAL ENGINEERING
TABLE 15.—PLATINUM GAUZE, 80-MESH, 0.003-iN. WIDE.   TEMPERATURES OBSERVED
THROUGH ONE WINDOW BY OPTICAL PYROMETER SIGHTED NORMAL TO SURFACE
OF GAUZE, vs. TRUE TEMPERATURE OF GAUZE
Observed temperature, degrees Centigrade	True temperature, degrees Centigrade	Observed temperature, degrees Centigrade	True temperature, degrees Centigrade
600	675	850	975
650	730	900	1,035
700	790	950	1,095
750	850	1,000	1,160
800	910	1,050	1,220
f a Window.—It is frequently necessary, especially in the laboratory,
i optical pyrometer into a furnace through a window.   What correction
ipplied to the observed temperatures to take account of the loss of
,ne window?   Kanolt has measured the transmission coefficient for a
3f ordinary glass windows at X = 0.65/j, and obtained a mean value of
Hence we have
1     1 _ log 0.904 _       nnnnn4fi tf-§-"9^88-------0.0000046
where # is the true absolute temperature of the source and S is the observed absolute temperature.   The following table is computed from the above formula:
TABLE 16.—CORRECTION TO OBSERVED TEMPERATURES FOR ABSORPTION OF LIGHT
BY A SINGLE CLEAN WINDOW
Observed temperatures, degrees Centigrade	Correction to add, degrees Centigrade	Observed temperatures, degrees Centigrade	Correction to add, degrees Centigrade
600	3.5	1,600	16.0
800	5.4	1,800	20.0
1,000	8.0	2,000	24.0
1,200	10.0	2,500	36.0
1,400	13.0	3,000	50.0
Flames and Smoke.—The optical pyrometer cannot be used satisfactorily when sighted through flames or smoke. Usually the presence of dense flames increases the temperature reading, and the presence of smoke clouds absorbs so much radiation that the pyrometer may read several hundred degrees low. The optical pyrometer can be used to measure the temperature of the slag in an open-hearth furnace but the flames prove a serious hindrance except during reversals when observations may be taken to advantage. In a cement kiln the dust, smoke and flames all combine to make the observations very untrustworthy. Carbon dioxide, water vapor and other invisible gases produce no effect.